An estimated 80 percent of the approximately 50 million postmenopausal women in the United States have complaints of disrupted sleep patterns characterized by sleep loss. Several studies show that a relative lack of estrogen interferes with optimal sleep and exogenous estrogen replacement improves sleep maintenance. However, it has been reported that less than 15 percent of postmenopausal women use estrogen replacement therapy. A growing body of evidence demonstrates that exercise improves self-reported sleep quality in postmenopausal women. The long-range goal of this research is to develop an exercise prescription that will improve the sleep of postmenopausal women. The specific objective of this research application is to determine the effects of a moderate intensity, home-based exercises program on objective sleep parameters, self-reported sleep quality, and daytime sleepiness. The central hypothesis is that a regular exercise program will improve the sleep architecture and sleep continuity patterns, subjective sleep quality, and daytime sleepiness in estrogen-deficient postmenopausal women. Employing an experimental, repeated measures, randomized treatment control group design, forty postmenopausal women who are estrogen deficient, sedentary, and have a sleep maintenance disorder will be randomly assigned to one of two conditions; an exercise group or a control group. The exercise will be home-based, moderate intensity walking with a frequency of 5 times per week and a duration of 30 minutes per session. Measurements of sleep architecture, sleep continuity, sleep quality and daytime sleepiness will be obtained at baseline and after the 16-week exercise or non-exercise period. Data on potential mediating variables of estradiol and circadian core body temperature rhythm will also be collected.